Straight chain polymeric molecules containing chlorine and hydrogen on alternating carbon atoms are known to dehydrohalogenate when exposed to ultraviolet light. This process begins with the generation of free radicals on the molecule with the subsequent formation of polyenes which absorb visible light. The use of this system for imaging is of only limited value since it requires extensive radiation to cause appreciable dehydrohalogenation. In order to provide a feasible photoimaging process, the above-described process must be accelerated so that the image is formed in a reasonable period of time upon irradiation.
Adding chemical components to the halogen containing polymer increases the rate of dehydrohalogenation. Owen and Bailey disclose in the Journal of Polymer Science, Vol. 10, (113-122, 1972) that benzophenone will photosensitize the degradation of polyvinyl chloride. This reference states that among the most important characteristics of benzophenone is its ability to abstract a hydrogen atom from a donor. It has also been proposed that a Friedel-Crafts catalyst be used to promote the dehydrohalogenation; however, this type of inorganic catalyst is not readily combinable with the organic polymer. A series of patents, i.e. U.S. Pat. Nos. 2,772,158; 2,789,052 and 2,789,053, disclose the use of Friedel-Crafts type catalyst progenitors, which are more compatible with the polymer, to solve this problem. Of course, such a system requires the inclusion of at least two separate species to form the Friedel-Crafts type catalyst in situ with the consequent increased difficulty in formulation inherent in such a two component system. In addition, the Friedel-Crafts progenitors proposed, e.g. aluminum stearate, tend to cloud the composition and provide, at best, films which are translucent.
It is disclosed by Kotov et al, In Doklady AN SSSR, Vol. 159, pp. 640-643 that polyvinyl chloride in combination with an electron acceptor such as chloranil will become colored upon irradiation with ionizing radiation, e.g. gamma or beta radiation, from 77.degree.-133.degree.K. However, this reference also discloses that the addition of acceptor impurities inhibits the coloring of the thawed polymer upon irradiation.
It is disclosed by Loan in Polymer Preprints, Vol. II, No. I, page 224, 1970, that tetracyanoethylene increases the rate of thermal dehydrochlorination of polyvinyl chloride with subsequent blackening of the polymer. These experiments were carried out at ca. 160.degree.C. and would not be suggestive of an imaging process due to the generalized nature of the darkening.
It would be desirable, and it is an object of the present invention, to provide a novel high gain imaging system.
It is another object to provide such a system which is based upon the dehydrohalogenation of a chlorine, bromine, or iodine containing polymer.
A further object is to provide such a system which is not hindered by free radical quenching as is the case with systems employing benzophenone as the dehydrohalogenation promotor.
An additional object is to provide a system which rapidly dehydrohalogenates upon exposure to actinic radiation.